Here is an email I sent to a friend who was asking about this verse in regards to Calvinism. I used this page, as well as some other resources, so thanks! I want to share it here so others can gain from it and perhaps take some things when they need to explain the verse to their friends!
Hey! I'm glad to hear from you.
I'll start with a bit about me and what I believe. So as you know I am a Calvinist, and I'd probably label myself as a Reformed Baptist as opposed to classic Reformed (Presbyterian). Some of the people I have learned from and hold in high regard as men of God are Charles Spurgeon, John Piper, James White, RC Sproul, Paul Washer, John MacArthur, and some oldies like Jonathan Edwards, John Owen, John Calvin, A.W Pink...when it comes to their views of God sovereignty, I would be in full agreement which means that I believe God is completely and absolutely sovereign over all things, meaning that I believe everything that has happened, is happening and will happen have been ordained or predetermined by God, and yes, that includes the Fall, the entrance of sin and death into the world. This doesn't mean I believe God is the author of sin though, because along with all those men I mentioned I understand that this is a paradox, how God can be completely sovereign and yet remain blameless and sinless. We can touch more on that later though.
I have taken a long time to get to this point, trust me. It didn't happen overnight. I had to wrestle with the Bible, with my own thoughts and objections and it was tough sluggin'. I was confronted with these powerful verses, and rather than suppress them or try to explain them away, I took the time to understand the implications of what it means for God to be completely sovereign and now that I am here, God's sovereignty is a hill I would die on. I fully believe these doctrines.
So anyways, enough about me, let's deal with the text you brought up, 2 Peter 3:9. Here it is in the ESV, to see a slightly different rendering:
"The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance."
For the most part it is the same as the KJV but I will point out a couple differences. The KJV says, "but is longsuffering to us-ward" whereas the ESV has "but is patient toward you". And then the KJV says, "not willing that any should perish" and the ESV, "not wishing that any should perish". Those are the two notable differences I saw, but they are minor and I don't think they alter the meaning whatsoever.
The first thing I want to get out there is that this verse is not just a problem for the Calvinist, but for anyone who believes that the consequences of dying without the forgiveness of sins is eternal punishment in hell. As Jim McClarty puts it, "the same God that is apparently not willing that any should perish also created hell...the same God that is not willing that any should ...